Past early childhood, injury to the central nervous system (CNS) results in functional impairments that are largely irreversible. Within the brain or spinal cord, damage resulting from stroke, trauma, or other causes can result in life-long losses in cognitive, sensory and motor functions, and even maintenance of vital functions. Nerve cells that are lost are not replaced, and those that are spared are generally unable to regrow severed connections, although a limited amount of local synaptic reorganization can occur close to the site of injury. Functions that are lost are currently untreatable.
Regenerative failure in the CNS has been attributed to a number of factors, which include the presence of inhibitory molecules on the surface of glial cells that suppress axonal growth; absence of appropriate substrate molecules such as laminin to foster growth and an absence of the appropriate trophic factors needed to activate programs of gene expression required for cell survival and differentiation.
By contrast, within the peripheral nervous system (PNS), injured nerve fibers can regrow over long distances, with eventual excellent recovery of function. Within the past 15 years, neuroscientists have come to realize that this is not a consequence of intrinsic differences between the nerve cells of the peripheral and central nervous system; remarkably, neurons of the CNS will extend their axons over great distances if given the opportunity to grow through a grafted segment of PNS (e.g., sciatic nerve). Therefore, neurons of the CNS retain a capacity to grow if given the right signals from the extracellular environment. Factors which contribute to the differing growth potentials of the CNS and PNS include partially characterized, growth-inhibiting molecules on the surface of the oligodendrocytes that surround nerve fibers in the CNS, but which are less abundant in the comparable cell population of the PNS (Schwann cells); molecules of the basal laminin and other surfaces that foster growth in the PNS but which are absent in the CNS (e.g., laminin); and trophic factors, soluble polypeptides which activate programs of gene expression that underlie cell survival and differentiation. Although such trophic factors are regarded as essential for maintaining the viability and differentiation of nerve cells, the particular ones that are responsible for inducing axonal regeneration in the CNS remain uncertain. As a result, to date, effective treatments for CNS injuries have not been developed.
Accordingly, methods and compositions for modulating the outgrowth of CNS neurons are still needed.
The present invention provides methods and compositions for modulating the axonal outgrowth of central nervous system neurons. The invention is based, at least in part, on the discovery that purine nucleosides and analogs thereof are capable of modulating (i.e. either stimulating or inhibiting) axonal outgrowth of CNS neurons. Accordingly, the method of the invention involves contacting central nervous system neurons with a purine nucleoside or analog thereof. In one aspect, the invention provides methods for stimulating outgrowth, preferably using inosine or guanosine nucleosides or analogs thereof. In another aspect, the invention provides methods for inhibiting outgrowth, preferably using a 6-thioguanine nucleoside. In a preferred embodiment, the methods of the invention modulate axonal outgrowth of retinal ganglion cells.
The invention also provides methods for stimulating the outgrowth of central nervous system neurons following damage or other injury to the CNS neurons (e.g., stroke, Traumatic Brain Injury, cerebral aneurism, spinal cord injury and the like). These methods involve administering to a subject a purine nucleoside (e.g., inosine or guanosine), or analog thereof, such that axonal outgrowth is stimulated. In one aspect, the purine nucleoside or analog thereof is administered by introduction into the central nervous system of the subject, for example into the cerebrospinal fluid of the subject. In certain aspects of the invention, the purine nucleoside or analog thereof is introduced intrathecally, for example into a cerebral ventricle, the lumbar area, or the cisterna magna. In a preferred embodiment, the stimulatory method of the invention promotes outgrowth of damaged retinal ganglion cells. The purine nucleoside or analog thereof can be administered locally to retinal ganglion cells to stimulate axonal outgrowth.
In another embodiment, the invention provides methods for inhibiting outgrowth of CNS neurons in which a purine nucleoside (e.g., 6-thioguanine) is administered to a subject. The inhibitory methods of the invention can be used to inhibit axonal outgrowth in, for example, neuroproliferative disorders or neuropathic pain syndromes.
In yet another aspect of the invention, the purine nucleoside or analog thereof is administered in a pharmaceutically acceptable formulation. The pharmaceutically acceptable formulation can be a dispersion system, for example a lipid-based formulation, a liposome formulation, or a multivesicular liposome formulation. The pharmaceutically acceptable formulation can also comprise a polymeric matrix, selected, for example, from synthetic polymers such as polyesters (PLA, PLGA), polyethylene glycol, poloxomers, polyanhydrides, and pluronics or selected from naturally derived polymers, such as albumin, alginate, cellulose derivatives, collagen, fibrin, gelatin, and polysaccharides.
In a further aspect of the invention, the pharmaceutically acceptable formulation provides sustained delivery, e.g., xe2x80x9cslow releasexe2x80x9d of the purine nucleoside to a subject for at least one, two, three, or four weeks after the pharmaceutically acceptable formulation is administered to the subject. Sustained delivery of a formulation of the invention may be provided by use of, for example, slow release capsules or an infusion pump.
The invention, finally, provides a pharmaceutical composition comprising a purine nucleoside or analog thereof and a pharmaceutically acceptable carrier.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.